Govt. engg. College, valsad Sub: Oc&up(2130501) Tala Manthan j. 140190105113 Guided by : - prof. B.R.Sudani*

Heterocyclic Compound: Pyridine Happy Tree (China)

PYRIDINE:- Pyridine is a six membered heterocyclic compound with molecular formula of C6H5N and it is obtained from coal tar. It may be formally derived from the structure of benzene through the exchange of one ring carbon for a sp2 hybridized nitrogen a nitrogen.

Pyridine is an aromatic compound, however, the nitrogen’s lone pair of electrons is in an sp2 orbital orthogonal to the p orbitals of the ring, therefore it is not involved in maintaining aromaticity but it is available to react with protons thus pyridine is basic.

Pyridine can be represented as a resonance hybrid of the following structures.

Synthesis of Pyridine: 1- From pyrrole: Pyrrole reacted with chloroform in presence of potassium hydroxide gives to pyridine.

Isolation of pyridine from coal-tar: Light oil fraction of coal-tar is treated with dilute H2so4 .This dissolves pyridine and other basic substances which form soluble sulphates. The acid layer is treated with NaoH when the bases are liberated. These are purified by rectification. The mixture of pyridine bases so obtained is used industrially in denaturing spirit, as well as a solvent in the purification separated from this mixture of pyridine bases by repeated fraction distillation.

Properties: 1. Physical: Pyridine is a colorless refractive liquid which has on unpleasant odour. It is miscible with water in all properties and is hygroscopic. It is a good solvent for most organic compounds and dissolved many inorganic salts.

2.chemical: Pyridine is basic in nature and resembles benzene in many of its properties. Its, however less reactive and is only very slowly attacked by boiling nitric acid or chromic acid. Important reaction of pyridine are given below:

1-Electrophilic substitution: The negative pole in pyridine ring is at N while the positive pole is at carbon skeleton which is opposite to what happens in pyrrole. This is due to the greater electronegativity of nitrogen (relative to carbons) it tends to withdraw the electron density from carbon atoms at positions 2, 4 and 6 which therefore acquire partial positive charges while the N atom acquires partial negative charge and the carbons at positions 3 and 5 (β-position) remain neutral therefore these positions are the most preferred for elctrophilic attack.

(a)Halogenation: At 570k in the presence of a catalyst. It forms a mixture of 3 bromo pyridine and 3,5-dibromo pyridine. Examples:

At 570k in the presence of a catalyst At 570k in the presence of a catalyst. It forms a mixture of 3 bromo pyridine and 3,5-dibromo pyridine.

(b)Nitration: On heating with concentration H2So4 and HNo3 at 270k, it gives 3-nitro pyridine.

(c)Sufonation: On heating with concentration H2So4 at 620k for some hours, it gives pyridine 3-sulphonic acid.

(d)Reduction: Pyridine on reduction with Na and ethanol gives piperidine electrolytic reduction using Ni also give piperidine.

Pyridine on reduction with Na and ammonia gives 1,5dihydropyridine is called Birch reduction, Also reduction with LiAlH4 or hydrogen ion and water gives 2,4-dihydro pyridine.

Nucleophilic substitution in pyridine: In pyridine, Nucleophilic substitution take place readily, particularly at the 2-and 4-positions. For examples, a halogen atom at the 2-and 4-positions is readily replaced by OH, CN, NH3 etc.

An important example of Nucleophilic substitution in pyridine is its amination by sodamide (Chichibabin reaction).

Reaction with KoH:

Uses: In reaction where in a halogen acid is produced or where molecule of halogen acid is to be removed to geunsaturated compounds, e.g. in acetylation, benzoylation or removal of H Br from bromo-succinic ester As a catalyst in many reactions, e.g. in the formation of a Grignard reagent, in perkin and knoevenagel condensation reactions.

As a solvent. In the manufacture of drug intermediates like vitamin B6 sulpha-pyridine etc. In denaturing alcohol and for exterminating plant pests.

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